1. Field of the Invention
The present invention relates to a liquid coolant for immersion cooling, and an electronic device using the coolant.
2. Description of the Related Art
When a heat generator is cooled by a direct immersion in a coolant, relationships between the difference .DELTA.T of the temperatures of the heat generator and the coolant (superheat; .degree.C.) and the heat flux (W/cm.sup.2) removed from a unit area of the heat generator by the coolant generally exist as shown in FIGS. 1A and 1B. As the temperature of the heat generator is elevated, the heat flux is increased in accordance with the .DELTA.T, but the coolant tends to bump and the heat flux is not increased when the phase shifts from the natural convection to the nucleated boiling phase, and then the nucleated boiling phase continues for a while and is shifted to a film boiling phase. The heat flux reaches the maximum when the nucleated boiling phase is being shifted to the film boiling phase.
It is known to use a coolant having a low boiling point, mixed with another coolant having a higher boiling point, for cooling by a direct immersion of a semiconductor element in the coolant.
The inventors investigated and developed methods of cooling a semiconductor element by a direct immersion in a liquid coolant, and these methods are disclosed or published in (1) "Evaporation Cooling Module for Semiconductor" (U.S. Pat. No. 4,704,658), (2) "Cooling Computers by Direct Immersing LSIs in Liquid", Nikkei Electronics No. 425, Jul. 13, 1987 p 167-176, (3) "Overheat Phenomena in Boiling Cooling", 1982 Autumn 43rd Applied Physics Society Conference Proceedings, Sep. 28-30, p 569, 29-F-3, (4) "Liquid Cooling Type Electronic Device" (Japanese Unexamined Patent Publication No. 59-125643), (5) "Immersion Cooling for High-Density Packaging" IEEE TRANSACTION ON COMPONENTS, HYBRIDS, AND MANUFACTURE TECHNOLOGY, vol. CHMT-12, No. 4, Dec. 1987, p 643-646, (6) "STUDIES ON IMMERSION COOLING FOR HIGH DENSITY PACKAGING" ISHM '87 Proceedings p 175-180, (7) "Cooling Technique for Semiconductor Element" Semiconductor Integrated Circuit Techniques 24th Symposium Conference Papers, Jun. 2-3, p 30-35, etc.
U.S. Pat. No. 4,704,658 describes freons C.sub.2 Cl.sub.3 F.sub.3 (b.p. 49.degree. C.), C.sub.5 F.sub.12 (b.p. 30.degree. C.), C.sub.6 F.sub.14 (b.p. 56.degree. C.), etc. as the coolant, and discloses cooling modules corresponding to FIGS. 10 to 18 attached to this specification.
Nikkei Electronics No. 425 states that fluorocarbons having a molecular weight of several hundreds, and chemically stable as a liquid coolant for a direct immersion cooling of LSIs, are a colorless transparent liquid and have a boiling point of 30.degree.-150.degree. C. (an example is a fluorocarbon having a boiling point of 56.degree. C.; p-fluorohexane) and discloses that a coolant mixture does not have a specific boiling point and does have a temperature range of boiling, and that overshoot can be reduced by combining a plurality of coolants; the minimum overshoot is obtained by mixing two fluorocarbons having boiling points of 56.degree. C. (p-fluorohexane) and 101.degree. C. (p-fluoro-2-octanone) in a ratio of 20:80.
1982 Autumn 43rd Applied Physics Society Conference Proceedings states that a mixture of two coolants having boiling points of 50.degree. C. and 100.degree. C. provides substantially no overheating, i.e., a deviation from the ideal starting point of the nucleated boiling.
Japanese Unexamined Patent Publication No. 59-125643 describes a coolant comprising two fluorocarbons having boiling points which are at least 10.degree. C. different from each other, in respective amounts of at least 10% by weight; specifically, fluorocarbons having boiling points of 50.degree. C. and 102.degree. C.
IEEE TRANSACTION ON COMPONENTS, HYBRIDS, AND MANUFACTURE TECHNOLOGY, vol. CHMT-12 describes a mixture of fluorocarbons having boiling points of 56.degree. C. and 102.degree. C. for minimizing overshoot.
ISHM '87 Proceedings states that perfluorocarbon C.sub.6 F.sub.14 (b.p. 56.degree. C.) is suitable as a coolant at room temperature, and that the heat flux of C.sub.6 F.sub.14 (b.p. 56.degree. C.) is 10 W/cm.sup.2 when the film boiling occurs.
Semiconductor Integrated Circuit Techniques 24th Symposium Conference Papers disclose that a freon has a cooling capability of 20 W/cm.sup.2 by boiling cooling, the maximum heat flux relates to the gasification heat of a liquid coolant, and the size of bubbles due to boiling of a coolant relates to a surface tension of the coolant, for example, 0.5 mm for C.sub.6 F.sub.14 (b.p. 56.degree. C.), and is small, i.e., 0.05 mm, when the coolant is liquid helium having a small surface tension of 0.12 dyne/cm, whereby a three-dimensional high density packaging is possible.
The present invention is based on the results of the above investigation, and the object of the present invention is to improve the maximum heat flux of a coolant, and a cooling capability at a unit area of a semiconductor element by a coolant, while the temperature of film boiling is maintained as low as a temperature range allowable for a semiconductor element.